1. Field of the Invention
The invention relates to an electric heating device, in particular as additional heating for motor vehicles, with PTC heating elements.
2. Description of the Related Art
For the employment in motor vehicles, in particular motor vehicles with optimised consumption engines, where a low amount of thermal energy arises, additional electric heaters for heating the passenger compartment and engine are used. The use of such additional electric heaters in a motor vehicle air conditioning system is schematically represented in FIG. 1. The air conditioning system sucks in outside air 2 via a fan 3. The intake air flows through an evaporator 6, a heat exchanger 7 and the electric heating device 1. Subsequently, the heated air 4 is conducted into the vehicle's passenger compartment via corresponding outflow means.
For an electric additional heating in motor vehicles, PTC heating elements that are in thermally conductive communication with radiator elements for dissipating the heat to the intake air are preferably employed. The overall arrangement of a layered structure of PTC heating elements and radiator elements is generally held in a clamping squeeze in the heating for increasing the efficiency of the heating. By the clamping, high electrical and thermal contacting of the PTC heating elements is achieved.
PTC heating elements are temperature-dependent semi-conductor resistors that are heated when current is supplied, the resistance of the heating elements increasing at the same time. Due to the self-regulating properties of the PTC heating elements, an overheating can be securely prevented.
The PTC heating elements employed in electric heaters consist of ceramics and have a flat, generally rectangular structural shape. Current is supplied to the ceramic disks via the large exterior surfaces facing each other. Simultaneously, heat is dissipated via these surfaces. The large surfaces of the PTC heating elements therefore have to have an electrically as well as thermally well connection to the adjacent surfaces. The rated heating power of an electric heating device is the heating power that is to be provided by the heating device under certain standard conditions (e.g. 300 kg/h air flow rate at 0° C.). The actually provided heating power of the heating device results from the sum of the heating powers of the PTC heating elements inserted in the heating device which in turn depends on the characteristic properties of each PTC heating element.
PTC heating elements are normally characterised by the electrical resistance at 25° C. (R25 value) and the temperature at which their resistance suddenly rises (transition temperature). The heating power of a PTC heating element is closely connected with the course of the temperature-resistance characteristic of a PTC heating element, as at a fixed voltage, the heating power only depends on the (temperature-dependent) electrical resistance of the PTC element. Under the above-stated standard conditions, one can therefore also state a “rated heating power” for an individual PTC heating element that is characterised by its rated values for the R25 value and the transition temperature.
For reasons of economy, exclusively PTC heating elements of the same characteristic (R25 value and transition temperature), i. e. of the same “rated heating power”, are used, and this not for one single heating or a certain type of heating, but preferably for all electric heaters of one manufacturer.
In general, the PTC heating elements of a heating are distributed to a plurality of separately selectable heat stages. Each of the heat stages is designed to achieve a certain rated heating power and therefore has a corresponding number of PTC heating elements.
Due to differences in the manufacture, the characteristic properties of a PTC heating element, and thus its actually delivered heating power, often significantly depart from the corresponding rated values. The usual departures of the R25 value range between 35% and 50%. The mean value of the departures differs with respect to the charges, i.e. from a closed amount of doped powder and sinter amount.
For taking into consideration the departures of the PTC heating elements from their rated heating power due to differences in the manufacture, the actual heating power of a heating/a heat stage can vary within a given tolerance range. In the worst case, in a heating/heat stage, only PTC elements having the same amount of departures from their rated heating power are employed. The individual departures can summarize in this case, such that the admissible tolerance limits of the heating/heat stage are exceeded or not achieved.
In order to avoid exceeding or falling below the tolerance limits, PTC heating elements with compensating departures are conventionally combined in one heating/heat stage. By means of such a combination of opposed departures, PTC heating elements can be used the departures of which are relatively high compared to the tolerances of the heating/heat stage. A disadvantage of this approach, however, is the selection and storage effort required for it. That means, first the individual departure of single PTC heating elements has to be established. Subsequently, a sufficient piece number of PTC heating elements of varying departures have to be stored for a continual production process.